In any center cutting drill, that part of the cutting edge closest to the center axis, called the chisel edge, is one of the most critical portions. Entire trade journal articles have been devoted just to the chisel edge and its shape. Being the most slowly moving part of the drill, the chisel edge produces a chip that is often not so much a discrete chip as it is a smeared, wiped extrusion of metal. Moreover, being located at the most crowded part of the drill, the chips produced by the chisel edges are more likely to pack and build up, and have a less direct exit path into the drill side flutes that take the chips away from the cutting interface and ultimately out of the hole. In a two flute drill, it is relatively easy to provide a chisel edge that operates well. The end of the drill is ground with two main cutting edges, each of which is defined by a plane, known as a rake face, that is ahead of a central, radial plane of the drill. The web of metal left between the two main cutting edges is ground in a four facet design in which a secondary clearance face behind each cutting edge extends past center just far enough to become the rake face of one of a pair of short chisel edges that bridge the web. A less than ideal result of this four facet design is that each chisel edge has a consequently negative axial rake angle. This means that the chisel edge is effectively "dull" and not "sharp" as it sees the workpiece, causing a scraping action, rather than a true cutting action. However, as viewed along the center axis, each chisel edge merges into a respective cutting edge smoothly. The chip that does form on the chisel edge, despite its relative dullness, does move smoothly out and radially along the chisel edge, always moving closer to its respective cutting edge, and without building up on or packing into any concavities or undercuts on the chisel edge. Because the chisel edge chips are plowed smoothly out from the center axis and into the side flute, they can be efficiently removed and flushed out of the hole, reducing drill thrust forces and drill wear. This is especially true for coolant feeding drills.
The great drawback of two flute drills is inherent in their name. There are only two cutting edges and chip exit flutes. More of each would obviously provide more cutting power, and more exit paths for the chips. Moreover, the nature of a two flute drill is such that a greater percentage of the drill body is ground away to create two flutes than would be the case for a three or four flute drill, which reduces its stiffness and resistance to bending. What is not obvious with a multi-flute (meaning more than two) drill design is how to bring more than two chisel edges together at the center axis of the drill and yet still achieve the smooth chisel edge chip removal action that is possible for the two flute drill. One solution for the four flute case is disclosed in U.S. patent application Ser. No. 07/832,513, assigned to the current assignee. There, a two flute drill type of chisel edge serves two of the four cutting edges, while the other two cutting edges are simply not brought all the way in to center. The design there does not lend itself to the three flute case, with its odd number of edges, however. Some other design would be necessary in order to bring three or more chisel edges all together at center.
An even greater challenge is presented when attempting to drill into a workpiece surface that is not perfectly flat. A conventional center cutting drill, with its single center point, will tend to "walk", and will be difficult to hold on axis. In response, so called "toothed" drill designs have been developed. Instead of a single, central point, three or more V-shaped cutting edges create an equal number of points or "teeth" that surround a generally concave center. Especially in the case of the three tooth design, a greater stability is achieved when a curved surface is drilled, since the three teeth contact the surface analogous to a three legged stool. However, there still must be some kind of a chisel edge, a "point within the points," at the very center, in order for there to be complete cutting. When the drill center point is recessed axially from the surrounding teeth, grinding access is far more limited.
One maker of toothed drills is a German company called Hartzmetall, whose patents show a couple of different chisel edge approaches. One design, shown in U.S. Pat. No. 4,594,034, simply avoids the issue by bringing only two of the three edges all the way in to center. Another design brings all the edges together at the center, in both a three and a four flute design, but the way in which the center point ground creates chisel edges that are less than satisfactory. The main V-shaped cutting edges lie directly on, rather than ahead of, a central, radial plane of the drill. A gashing wheel is used to grind gashes near the center of the drill, one for each tooth. The gashing wheel has a flat side and a curved rim, and is sent along a tangent line path through the drill center point, with the flat side of the wheel held on the same radial plane as the cutting edges. The result is that a sharp, pyramid shaped "spur" is left at the center with curved edges that merge into and provide the chisel edges for the V-shaped cutting edges.
There are several drawbacks to this design. The center spur is too sharp, and consequently weak, for many drilling applications. Furthermore, the axial rake angle of the cutting edge shifts from positive to negative at the chisel edge. The patent explicitly recognizes this, describing the cutting action as being transformed, at the point where the positive to negative shift occurs, into a "scraping" action. An attempt is made to portray this as an advantage, with the resulting increased cutting force at the spur being said to help hold the drill on axis. However, realistically speaking, it is the V-shaped teeth that engage the workpiece first, and it is they which will have to provide the initial centering action of the drill. More fundamental is a problem not articulated in the patent, but which would soon show up in use. Since the straight cutting edge lies in a central or radial plane, each chisel edge, after it curves out and away from the center axis, must curve back in order to merge with its respective straight cutting edge, as viewed looking down the center axis. A "cup" or undercut is thereby created, meaning that, as one moves radially out along the chisel edge, it changes slope at some point, relative to the straight cutting edge. The chisel edge chip would not be plowed smoothly and continuously out and along such a chisel edge. Rather, it would tend to build up and "catch" on the chisel edge. An analogy would be the blade of a snow plow having a large dent at some point along its edge. Snow would eventually move from the outer to the inner corner of the blade, but would collect and build up at the dent. It would not be a problem if the blade edge curved continuously along its length, so long as it did not change slope and curve back at some point.
This, then, is the somewhat extensive background of the current invention. There is, despite the plethora of commercially available drills, an apparently unmet need for a multi-tooth, center cutting drill design in which the central, chisel edges are all complete, but all merge smoothly and continuously into their cutting edges. Ideally, the chisel edges would also have an axial rake angle that was everywhere positive and "sharp", while the center peak of the drill would not be so sharp as to be weak.